In the prior art, it is known to separate plasmids present in various bacteria, by centrifugation. The centrifugation of bacterial cells, only enables a low yield of plasmids to be obtained, of the order of 20%. Now, plasmids have, at the present time, many uses, particularly as hybridization probes or as vectors. As the demand for plasmids is becoming greater and greater, the possibility of obtaining whole plasmids with very high yield, is therefore of positive practical interest to the industries concerned.